Memory system with reversible resistivity-switching using pulses of alternate polarity

ABSTRACT

A memory system includes a plurality of non-volatile storage elements that each comprise a diode (or other steering device) in series with reversible resistance-switching material. One or more circuits in the memory system program the non-volatile storage elements by changing the reversible resistance-switching material of one or more non-volatile storage elements to a first resistance state. The memory system can also change the reversible resistance-switching material of one or more of the non-volatile storage elements from the first resistance state to a second resistance state by applying one or more pairs of opposite polarity voltage conditions (e.g., pulses) to the respective diodes (or other steering devices) such that current flows in the diodes (or other steering devices) without operating the diodes (or other steering devices) in breakdown condition.

BACKGROUND

1. Field

This application relates to technology for non-volatile storage.

2. Description of the Related Art

A variety of materials show reversible resistance-switching behavior,and as such may be suitable for use with memory systems. For example,transition metal oxides have been proposed for reversibleresistance-switching memories. Upon application of sufficient voltage,current, or other stimulus, the reversible resistance-switching materialswitches to a stable low-resistance state, which is sometimes referredto as SETTING the device or performing a SET operation. Thisresistance-switching is reversible such that subsequent application ofan appropriate voltage, current, or other stimulus can serve to returnthe reversible resistance-switching material to a stable high-resistancestate, which is sometimes referred to as RESETTING the device orperforming a RESET operation. This conversion can be repeated manytimes. The low resistance state is sometimes referred to as an “on”state. The high resistance state is sometimes referred to as an “off”state. For some reversible resistance-switching materials, the initialstate is low-resistance rather than high-resistance. For purposes of thefollowing discussion, any of the operations of RESETTING and SETTING maybe considered to be a programming operation. In some devices, prior tobeing able to perform SET operations and RESET operations, a devicesmust be initialized by applying a voltage potential across thereversible resistance-switching material in an operation referred to asFORMING.

These reversible resistance-switching materials are of interest for usein nonvolatile memory systems. One type of memory system is referred toas a cross-point array, which is a matrix of memory elements typicallyarranged along x-axes (e.g., word lines) and along y-axes (e.g., bitlines). A digital value may be stored as a memory resistance (high orlow). The state of a memory cell can be read by supplying appropriatevoltages to the bit line and word line connected to the selected memorycell. The state of the memory cell can be read as an output voltage ofthe bit line connected to the selected memory cell. One resistance statemay correspond to a data “0,” for example, while the other resistancestate corresponds to a data “1.” Some switching materials may have morethan two stable resistance states.

Operating memory devices that employ reversible resistance-switchingmaterials is challenging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified perspective view of one embodiment of a memorycell with a steering element.

FIG. 2 is a graph depicting voltage and current characteristics for adiode.

FIG. 3A-E depict voltage pulses used to program memory cells.

FIG. 4 s a flow chart describing one embodiment of a process foroperating a memory system.

FIG. 5 s a flow chart describing one embodiment of a process forprogramming one or more memory cells.

FIG. 6 is a simplified perspective view of a portion of a first memorylevel formed from a plurality of the memory cells of FIG. 1.

FIG. 7 is a simplified perspective view of a portion of a threedimensional memory array.

FIG. 8 is a simplified perspective view of a portion of a threedimensional memory array.

FIG. 9 is a block diagram of one embodiment of a memory system.

FIG. 10 depicts a circuit that can read the state of a memory cell.

FIG. 11 is a flow chart describing one embodiment of a process forprogramming one or more memory cells.

FIG. 12 depicts a portion of a memory array and the signals used toprogram a selected memory cell.

FIG. 13 is a flow chart describing one embodiment of a process forprogramming one or more memory cells.

FIG. 14 depicts a portion of a memory array and the signals used toprogram a selected memory cell.

FIG. 15 is a flow chart describing one embodiment of a process forprogramming one or more memory cells.

FIG. 16 depicts a portion of a memory array and the signals used toprogram a selected memory cell.

DETAILED DESCRIPTION

A memory system is disclosed that includes a plurality of memory cells.In one embodiment, each memory cell comprises a steering device inseries with reversible resistance-switching material; however, otherstructures can also be utilized. The memory cells are programmed byperforming SET and/or RESET operations. Depending on the materials usedfor the steering device and the reversible resistance-switchingmaterial, one of the SET and RESET operations is performed by applying afirst voltage to the memory cells wherein the steering device (e.g.,diode) is forward biased. The other of the SET and RESET operations canbe performed by applying one or more pairs of opposite polarity voltageconditions to the memory cells. One example of opposite polarity voltageconditions includes a first voltage pulse that forward biases thesteering element and a second voltage pulse that reverse biases thesteering device.

Memory Cell

Prior to discussing details of programming a memory cell, an examplememory cell and system will be discussed. FIG. 1 is a simplifiedperspective view of one embodiment of a memory cell 1 which includes areversible resistivity-switching element 2 coupled in series with asteering element 4 between a first conductor 6 and a second conductor 8.In some embodiments, the steering element 4 is a diode. In oneembodiment, the diode steering element 4 is a p-i-n diode. For example,diode 4 may include a heavily doped n+ polysilicon region 42, a lightlydoped or an intrinsic (unintentionally doped) polysilicon region 44above the n+ polysilicon region 42, and a heavily doped p+ polysiliconregion 46 above the intrinsic region 44. In some embodiments, a thin(e.g., a few hundred angstroms or less) germanium and/orsilicon-germanium alloy layer (not shown), with about 10% or more ofgermanium when using a silicon-germanium alloy layer, may be formed onthe n+ polysilicon region 42 to prevent and/or reduce dopant migrationfrom the n+ polysilicon region 42 into the intrinsic region 44, asdescribed, for example, in U.S. Patent Application Publication No.2006/0087005, filed Dec. 9, 2005 and titled “DEPOSITED SEMICONDUCTORSTRUCTURE TO MINIMIZE N-TYPE DOPANT DIFFUSION AND METHOD OF MAKING,”which is hereby incorporated by reference herein in its entirety. Itwill be understood that the locations of the n+ and p+ regions may bereversed.

In another embodiment, the diode steering element 4 is a punch-thrudiode. A punch-thru diode used as a steering element may be may be aN+/P−/N+ device or a P+/N−/P+ device. In another embodiment, the diodesteering element 4 is a Schottky diode. In another embodiment, the diodesteering element 4 is a back-to-back Schottky diode. In someembodiments, diode 4 may be formed from a polycrystalline semiconductormaterial such as polysilicon, germanium, or another semiconductor. Also,the diode steering element 4 may comprise more than one type ofsemiconductor. For example, diode 4 may be formed from a polycrystallinesilicon-germanium alloy, polygermanium or any other suitable combinationof semiconductors. In some embodiments, each region 42, 44, 46 of thediode steering element 4 is formed from the same material (but dopeddifferently). However, it is not required that each region be formedfrom the same material. For example, a heterostructure may be possible.

Steering element 204 is not limited to being a diode. In one embodiment,the steering element 4 is a transistor. For example, a Field EffectTransistor (FET) can be used for the steering element 4.

Memory cell 1 has a reversible resistivity-switching element thatincludes a reversible resistivity-switching material 30, an upperelectrode 32, and a lower electrode 34. Electrode 32 is positionedbetween reversible resistivity-switching material 30 and conductor 8. Inone embodiment, electrode 32 is made of TiN. Electrode 34 is positionedbetween reversible resistivity-switching material 30 and steeringelement 4. In one embodiment, electrode 34 is made of titanium nitride,and may serve as a barrier layer

Memory cell 1 has an electrode 13 at the bottom of the memory cell 1 tofacilitate electrical contact between the steering element 4 and othercircuit elements. In one embodiment, electrode 13 is formed from TiN.Note that the relative positions of the steering element 4 and thereversible resistivity-switching element 2 could be reversed. Forexample, the steering element 4 could be above the reversibleresistivity-switching element 2.

Reversible resistivity-switching element 2 includes reversibleresistivity-switching material 30 having a resistance that may bereversibly switched between two or more states. For example, thereversible resistivity-switching material may be in an initialhigh-resistance state upon fabrication that is switchable to alow-resistance state upon application of a first physical signal. Forexample, the reversible resistivity-switching element 2 may switchstates in response to application of a first amount of energy, charge,heat, voltage, current or other phenomena. Application of a secondamount of energy, charge, heat, voltage, current or other phenomena mayreturn the reversible resistivity-switching material to thehigh-resistance state. Alternatively, the reversibleresistivity-switching element may be in an initial low-resistance stateupon fabrication that is reversibly switchable to a high-resistancestate upon application of the appropriate energy, charge, heat, voltage,current or other phenomena. When used in a memory cell, one resistancestate may represent a binary “0” while another resistance state mayrepresent a binary “1.” However, more than two data/resistance statesmay be used. Numerous reversible resistivity-switching materials andoperation of memory cells employing reversible resistivity-switchingmaterials are described, for example, in U.S. Patent ApplicationPublication 2006/0250836, incorporated herein by reference in itsentirety.

In some embodiments, reversible resistivity-switching material 30 may beformed from a metal oxide. Various different metal oxides can be used.The metal-oxide may be a transition metal-oxide. Examples ofmetal-oxides include, but are not limited to, NiO, Nb₂O₅, TiO₂, HfO₂,Al₂O₃, MgO_(x), CrO₂, VO, BN, and AlN. In one embodiment, the memoryelement electrodes 32 and 34 are formed from TiN. More information aboutfabricating a memory cell using reversible resistivity-switchingmaterial can be found in United States Patent Application Publication2009/0001343, filed on Jun. 29, 2007, entitled “Memory Cell that Employsa Selectively Deposited Reversible Resistance Switching Element andMethods of Forming the Same,” which is hereby incorporated herein byreference in its entirety. In some embodiments a bipolar metal oxideswitching element is used. In some of those embodiments, the electrodematerials may be different in work function or other properties and maybe chosen to form a switching element with a preferential pulse polarityfor SET opposite the preferred pulse polarity for RESET. In one exampleof a bipolar embodiment, the top electrode is TiN and the bottomelectrode is n+ doped polysilicon and the switching element comprises aHfOx layer on the bottom and a TiOx buffer layer on top.

Conductors 6 and 8 may include any suitable conductive material such astungsten, any appropriate metal, heavily-doped semiconductor material, aconductive silicide, a conductive silicide-germanide, a conductivegermanide, or the like. In the embodiment of FIG. 1, conductors 6 and 8are rail-shaped and extend in different directions (e.g., substantiallyperpendicular to one another). Other conductor shapes and/orconfigurations may be used. In some embodiments, barrier layers,adhesion layers, antireflection coatings and/or the like (not shown) maybe used with conductors 6 and 8 to improve device performance and/or aidin device fabrication.

FIG. 1 shows a memory cell 1 in a cylindrical shape and conductors inthe shapes of rails. However, the technology described herein is notlimited to any one specific structure for a memory cell. Otherstructures can also be used to form memory cells that include reversibleresistivity-switching material. For example, the following patentsprovide examples of structures of memory cells that can be adapted touse reversible resistance-switching material: U.S. Pat. Nos. 6,952,043;6,951,780; 6,034,882; 6,420,215; 6,525,953; and 7,081,377.

As discussed above, a memory cell can be programmed by performing a SETor RESET operation. In one embodiment, both the SET and RESET operationsare performed by applying appropriate voltages. For example, a SETvoltage can be applied to SET a memory cell and a RESET voltage can beapplied to RESET the memory cell. In one example implementation, the SETvoltage is applied as a voltage pulse and the RESET voltage is alsoapplied as a voltage pulse. Note that although an ideal pulse may have arectangular shape, the use of the term pulse can include a broadervariety of shapes that approximate the real-world behavior of a signalramped up and ramped down from a maximum magnitude. In unipolaroperation, both the SET voltage and RESET voltage can be accomplishedusing pulses of positive voltages. With bipolar operation, one of theSET and RESET operations will use a positive voltage pulse and the otherof RESET and SET operations will use a negative pulse. Instead of apulses, other voltage conditions can be used.

For bipolar switching, a relatively high current is often required forboth the SET operation and RESET operation. When a positive voltage isapplied across a diode being used a steering element in a memory cell,the diode will be forward biased; therefore, current will flow throughthe diode. For an ideal diode, when a negative voltage is applied to thediode, no current will flow through the diode. Therefore, to accomplisha SET or RESET operation using a negative voltage, some implementationswill cause a diode to go into the breakdown mode of operation. With somediodes, if a large enough negative voltage is applied across a diode,the pn junction will break down and a reverse current will flow throughthe diode. However, operating the diode in the breakdown mode increasesthe chances of device failure or degradation.

As discussed above, when a reverse bias (e.g., negative voltage) isapplied, the ideal diode will not conduct any current. However, anactual implementation of a diode will experience some leakage currentwhen reverse biased. In most implementations, designers of electricalcomponents seek to minimize the reverse leakage current. Whileminimizing conventional reverse leakage remains the goal, the technologydescribed herein will take advantage of the much higher reverse current,when the device is reverse biased, under certain transient conditions,in order to program the memory cell using that reverse current. Thus,one programming mode (either SET or RESET) will use a forward biasvoltage condition to perform the programming operation while the otherprogramming mode will use a reverse bias voltage condition to program(the other of SET or RESET) using the reverse current that results fromthe reverse bias voltage applied across the diode.

In order to program the reversible resistance switching element 2 ofFIG. 1, a large enough current must be present. The conventional reverseleakage current that can be, for instance, in pico Amp to nano Amp rangeis typically not sufficient. Therefore, the reverse bias current shouldbe increased without putting the diode in breakdown mode. The proposedsolution to obtain enough reverse current without putting the diode inbreakdown mode is to use a forward bias (with respect to the diode) tostore charge in the diode and then use a reverse bias to achieve highamplitude reverse transient current (referred to as diode reverserecovery current), at lower than breakdown voltage. Therefore, in oneexample implementation, the system will apply transient pulses ofalternate polarity to the memory cells. Depending on the reversibleresistance-switching material, the pulses of alternate polarity can beused for SET or RESET, with the other programming operation receivingone or more pulses of positive voltage.

FIG. 2 is a graph showing the IV characteristics of a diode over time.Line 80 plots voltage versus time for the diode V=V(t). Line 82 plotscurrent versus time I=I(t). The voltage VF represents the forwardbiasing voltage for the diode. The value IF represents the currentflowing when the diode is forward biased. The voltage VR represents avoltage that puts the diode in reverse bias (e.g. negative voltage withrespect to the diode). The current IRmin represents the current flowingthe diode at nearly steady state and represents what is typically calleddiode reverse leakage when the diode is reverse biased. In practice,Imin may be in the nano Amp range or lower. It has been found thatduring forward bias, a diode will store charge. When the voltage acrossthe diode is quickly transitioned from a forward bias to a reverse bias,there will be high reverse current IRM (diode reverse recovery current)for a small period of time ts (called high reverse conductivity stage).Subsequently, the current will be reduced to a steady state leakage(approaching the level of steady state reverse leakage e.g. of pico- tonano amps) during recovery time period trr (reverse recovery stage).During the time period ts+trrr, (total reverse recovery time) diodereverse recovery current will flow through the diode and, therefore,through the reversible resistance-switching material that is in serieswith the diode. If the magnitude of VR is greater than the magnitude ofVF, and VR is less than the breakdown voltage for the diode, then thecurrent IRM can potentially be higher than the current IF, depending ondiode parameters. In such a case, a high reverse current during diodereverse recovery (ts+trr) may be achieved.

One explanation for the diode reverse recovery current is that when thevoltage bias across the diode is reversed, the holes and electrons thatare injected and accumulated in the effective intrinsic region of thediode (in intrinsic region and lower doped parts of the N+ and P+emitters) during the forward pulse, are extracted, which dissipates thecharge in the intrinsic region. During that extraction and recombinationprocess, the diode will conduct in the reverse direction. To increasethe diode reverse recovery current to allow for programming thereversible resistance-switching material, it is beneficial to slow downthe recombination rate. Since one of the key recombination mechanisms inpoly silicon is trap-assisted recombination, the overall recombinationrate can be reduced by reducing trap density. One means for reducingtrap density is to improve polysilicon crystallinity. The goal is to getas close to a single grain diode as possible. That is, the design of thediode will seek to increase the grain size, with a single crystal diodeas the goal. Crystalline silicon which has one grain (and, therefore, nograin boundaries) is desired. Reducing the number of grains reduces thedefect/trap density, which can reduce recombination rate. In many otherapplications circuit designers want to speed up the recovery process ina diode; however, the technology described herein seeks to slow down therecovery and take advantage of that recovery. Strategies to increasegrain size include silicidation, thermal annealing in an inert gas, andpassivation. As a significant side benefit, reducing the trap densityand overall recombination rate will also increase diode forward currentwhich is beneficial for RRAM applications.

To obtain the desired diode reverse recovery current, a sufficientforward biasing voltage is used to cause the diode to store enoughcharge. Increasing the P+ and N+ doping level will allow more carriersto be injected in the intrinsic region during the forward bias toincrease the charge stored.

FIG. 3A shows a graph with two pulses 102 and 104. Pulse 102 representsa positive voltage pulse and pulse 104 represents a negative voltagepulse. Pulses 102 and 104 represent prior art bipolar operation. Forexample, pulse 102 can be used to SET a memory cell by applying aforward bias and pulse 104 can be used to RESET the same memory cell byapplying a reverse bias. Alternatively, pulse 104 can be used to SET thememory cell and pulse 102 can be used to RESET the memory cell.

The technology described herein, proposes a new scheme for bipolaroperation whereby one of the programming operations (either SET orRESET) is performed by using a positive voltage condition (e.g.,positive voltage pulse), and the other programming operation (either SETor RESET) is performed by applying one or more pairs of oppositepolarity voltage conditions to the reversible resistance-switchingmemory cell. FIG. 3B provides a graphic example of this technique. Forexample, positive voltage pulse 110 is an example of a voltage conditionwhich can be used for a RESET operation. When applying positive voltagepulse 110, the diode (other steering device) of the memory cell will beforward biased. Therefore, current will flow through the diode andthrough the reversible resistance-switching material. If the appropriatevoltage is applied, the programming operation will be performed. The SEToperation is performed by applying the pair of pulses 112 and 114.

The pair of opposite polarity voltage conditions depicted in FIG. 3Bincludes positive voltage pulse 112 and negative voltage pulse 114.Positive voltage pulse 112 provides a forward bias (a first polarity) tothe steering element 4 such that current will flow through reversibleresistance-switching material 2 and steering element 4 will store someamount of charge. Negative voltage pulse 114 will provide a reverse bias(second polarity) to steering element 4 such that steering element 4will experience some diode reverse recover current that will be used tochange the resistance state of reversible resistance-switching material2.

In the example depicted in FIG. 3B, positive voltage pulse 112immediately transitions into negative voltage pulse 114. This means thatthere is no time gap between the two pulses such that the second pulseis applied immediately after the first pulse. Additionally, the end ofpulse 112 becomes the beginning of pulse 114. Thus, pulse 112transitions into pulse 114.

In other embodiments, the SET operation is performed by the forwardbiasing voltage condition and the RESET operation is performed by theapplication of one or more pairs of opposite polarity voltage conditionsbeing applied to the resistance-switching memory cell.

If the voltage waveform of FIG. 3B is applied to a reversibleresistance-switching memory cell (e.g., memory cell 1 of FIG. 1), thenthe reversible resistance-switching memory cell will be changed to afirst resistance state in response to voltage pulse 110. Subsequently,the reversible resistance-switching memory cell will be changed from thefirst resistance state to a second resistance state by applying the pairof opposite polarity voltage conditions (e.g., positive voltage pulse112 and negative voltage pulse 114).

FIG. 3C shows the waveform for pulses 112 and 114 superimposed onwaveforms 120 and 122, which depict the current flowing through thediode as a result of the voltage pulses.

In the embodiment of 3B, only one positive voltage pulse 110 is depictedfor transitioning the memory cell to a first resistance state and onlyone pair of opposition polarity voltage pulses are depicted fortransitioning reversible resistance-switching memory cell from the firstresistance state to the second resistance state. In other embodiments,more than one pulse can be used to change the reversibleresistance-switching memory cell to the first state and more than onepair of opposite polarity voltage pulses can be used to transition thereversible resistance-switching memory cell from the first resistancestate to the second resistance state. For example, FIG. 3D shows threepositive pulses 130 being used to transition the memory cell to thefirst resistance state and three pairs of opposite polarity voltagepulses 132 being used to transition to the reversibleresistance-switching memory cell from the first resistance state to thesecond resistance state. Although three pulses are depicted, more orless than three pulses can be used for each programming operation.

In some embodiments, the system will perform some amount of programming,followed by verification of whether the programming was successful. Ifthe programming was successful, then no more programming needs to beperformed. If the programming was not successful, additional programmingwill be performed. In such a case, the system may apply a series ofvoltage conditions (e.g., pulses), with verification being performedbetween the application of the voltage conditions (e.g., pulses). Forexample, FIG. 3E shows positive pulses 140, 142 and 144, each designedto perform a programming operation. In between each of the programmingpulses 140, 142 and 144, the system will verify whether the device hascompleted the intended programming. Once the device has completed theintended programming, no more pulses will be applied.

FIG. 3E also shows three pairs of opposite polarity voltage pulses150/152, 154/156 and 158/160. Between each pair of opposite polarityvoltage pulses, the system will perform a verification process to seewhether the reversible resistance-switching memory cell has properlyprogrammed. The graph of FIG. 3E shows one pair of opposite polaritypulses being performed between verification processes. In someembodiments, multiple pulses or multiple pairs of pulses of oppositepolarity will be applied between verification processes. For example,the system may apply three of more pairs of opposite polarity voltagepulses (as shown in FIG. 3D), then verify, then repeat the three or morepairs of opposite polarity voltage pulses, etc. With each pair ofopposite polarity voltage pulses (or other voltage condition), thepositive pulse will cause the diode to be in forward bias and willcharge the diode, and the negative pulse will cause the diode totransition to reverse bias such that the reverse recovery current willflow for a short period of time with enough magnitude to perform theprogramming operation without causing the diode to operate in breakdowncondition.

FIG. 4 is a flowchart describing one embodiment of a process foroperating a memory system that can be programmed as discussed above withrespect to FIGS. 3B-3E. Some reversible resistance switching materialsmust be FORMED prior to programming. Typically, the forming processincludes applying a voltage with a magnitude higher than the voltagenecessary for SET or RESET. The forming process prepares a material tobe able to reliably switch resistance states as described herein. Step164 of FIG. 4 includes performing a FORMING operation. In step 166, oneor more memory cells can be programmed by changing their resistancestates, as described above. In one embodiment, step 166 includesapplying a single polarity of voltage conditions. In step 168, one ormore memory cells can be read one or more times. In some embodiments,step 168 is optional. In step 170, one or more memory cells areprogrammed by changing the resistance state. The process of programmingat step 170 includes applying one or more pairs of opposite polarityvoltage conditions as described above. For example, programming of step166 can utilize positive voltage pulses 130 and the programming of step170 can use the multiple pairs of opposite polarity voltage pulses 132.In one embodiment, step 166 can be used to SET memory cells and step 170can be used to RESET memory cells. In other embodiments, step 166 can beused to RESET memory cells and step 170 can be used to SET memory cells.In some embodiments, steps 166 and 170 can be performed concurrently,while in other embodiments they are performed at different times. Instep 172, one or more memory cells can be read one or more times. Insome embodiments, step 172 is optional. Thereafter, memory cells cancontinue to be programmed (steps 166 and 170) or read (step 168 and172), randomly, serially or in any order suitable for the application.

FIG. 5 is a flowchart describing one embodiment of the processes forprogramming memory cells. For example, the process of FIG. 5 can be usedto implement steps 166 and/or step 170. In step 180 of FIG. 5,programming is applied to the selected memory cell. That is, one or morepositive voltage pulses (e.g., pulses 130) can be applied or one or morepairs of opposite polarity voltage pulses (e.g., pulses 132) can beapplied, depending on which programming operation is being performed. Insome embodiments, one memory cell is programmed at a time. In otherembodiments, multiple memory cells can be concurrently programmed. Instep 182, the memory cells selected for programming are verified todetermine whether programming is complete. If the programming iscomplete (step 184), then the process is finished and reports a status(e.g., pass) in step 186. If the program is not complete, then it isdetermined whether the number of iterations of programming is more thana predefined limit (step 188). If the number of programming iterationsis not more than a predefined limit, then the process will perform thenext program iteration by looping back to step 180 and applyingadditional programming. If the number of programming iterations isgreater than the predetermined limit, then the process has failed andwill report its failure in step 190. One example of the predeterminedlimit for the number of programming iterations is five. If the status isfail (step 190), system control logic 330 on the memory chip or a hostdevice may take further corrective action such as using redundant memorycells Error correcting logic or recovery pulses applied to the failedcell.

Memory System

The above-described reversible resistance-switching memory cell, withthe above described programming process, can be used in a non-volatilestorage system. FIG. 6 is a simplified perspective view of a portion ofa first memory level 214 formed from a plurality of the memory cells 1of FIG. 1 that can be programmed as described above. For simplicity, thereversible resistivity-switching element 2 and the steering element 4are not separately shown. The memory array 214 is a “cross-point” arrayincluding a plurality of first conductors 6 (e.g., bit lines) and aplurality of second conductors 8 (e.g., word lines) between whichmultiple memory cells are coupled (as shown). Other memory arrayconfigurations may be used, as may multiple levels of memory.

FIG. 7 is a simplified perspective view of a portion of a monolithicthree dimensional array 216 that includes a first memory level 218positioned below a second memory level 220. In the embodiment of FIG. 7,each memory level 218 and 220 includes a plurality of memory cells 1 ina cross-point array. It will be understood that additional layers (e.g.,an inter-level dielectric) may be present between the first and secondmemory levels 218 and 220, but are not shown in FIG. 7 for simplicity.Other memory array configurations may be used, as may additional levelsof memory. In the embodiments of FIGS. 6 and 7, all steering devices(e.g., diodes) may “point” in the same direction, such as upward ordownward depending on whether p-i-n diodes having a p-doped region onthe bottom or top of the diode are employed, simplifying diodefabrication.

In some embodiments, the memory levels may be formed as described inU.S. Pat. No. 6,952,030, “High-Density Three-Dimensional Memory Cell,”which is hereby incorporated by reference herein in its entirety. Forinstance, the upper conductors 8 of a first memory level 218 may be usedas the lower conductors 8 of a second memory level 220 that ispositioned above the first memory level 218 as shown in FIG. 8. In suchembodiments, the diodes on adjacent memory levels preferably point inopposite directions, as described in U.S. patent application Ser. No.11/692,151, filed Mar. 27, 2007 and titled “Large Array Of UpwardPointing P-I-N Diodes Having Large And Uniform Current,” which is herebyincorporated by reference herein in its entirety. For example, thediodes of the first memory level 218 may be upward pointing diodes,while the diodes of the second memory level 220 may be downward pointingdiodes, or vice versa.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as a wafer, withno intervening substrates. The layers forming one memory level aredeposited or grown directly over the layers of an existing level orlevels. In contrast, stacked memories have been constructed by formingmemory levels on separate substrates and adhering the memory levels atopeach other, as in Leedy, U.S. Pat. No. 5,915,167, “Three DimensionalStructure Memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

FIG. 9 is a block diagram that depicts one example of a memory system300 that can implement the technology described herein. Memory system300 includes a memory array 302 that can be a two or three dimensionalarray of memory cells as described above. In one embodiment, memoryarray 302 is a monolithic three dimensional memory array. The arrayterminal lines of memory array 302 include the various layer(s) of wordlines organized as rows, and the various layer(s) of bit lines organizedas columns. However, other orientations can also be implemented.

Memory system 300 includes row control circuitry 320, whose outputs 308are connected to respective word lines of the memory array 302. Rowcontrol circuitry 320 receives a group of M row address signals and oneor more various control signals from System Control Logic circuit 330,and typically may include such circuits as row decoders 322, arrayterminal drivers 324, and block select circuitry 326 for both read andprogramming (e.g., SET and RESET) operations. Memory system 300 alsoincludes column control circuitry 310 whose input/outputs 306 areconnected to respective bit lines of the memory array 302. Columncontrol circuitry 306 receives a group of N column address signals andone or more various control signals from System Control Logic 330, andtypically may include such circuits as column decoders 312, arrayterminal receivers or drivers 314, block select circuitry 316, as wellas read/write circuitry, and I/O multiplexers. In one embodiment columndecoder 312 is a reversible polarity decoder circuit. In one embodimentrow decoder 322 is a reversible polarity decoder circuit. In oneembodiment, a reversible polarity decoder circuit has active low outputin one mode and active high output in another mode. Further details ofreversible polarity decoder circuits are described in U.S. Pat. No.7,542,370, filed on Dec. 31, 2006, which is hereby incorporated hereinin its entirety.

System control logic 330 receives data and commands from a host andprovides output data to the host. In other embodiments, system controllogic 330 receives data and commands from a separate controller circuitand provides output data to that controller circuit, with the controllercircuit communicating with the host. System control logic 330 mayinclude one or more state machines, registers and other control logicfor controlling the operation of memory system 300.

Integrated circuits incorporating a memory array usually subdivide thearray into a number of sub-arrays or blocks. Blocks can be groupedtogether into bays that contain, for example, 16, 32, or a differentnumber of blocks. Each block in a monolithic three dimensional memorymay have many layers of memory cells. For example, a block might include8 layers. Each layer may include hundreds, or thousands of bit lines andword lines. For example, a layer might have about a thousand bit linesand about 8 thousand word lines. In some implementations, there is a bitline driver associated with each bit line. Note that a given drivercould be shared between two or more bit lines. Also note that it is notrequired that a given bit line have only one driver associatedtherewith. In some implementations, some of the drivers are physicallylocated on one end of the bit lines and other drivers on the other endof the bit lines.

As frequently used, a sub-array is a contiguous group of memory cellshaving contiguous word and bit lines generally unbroken by decoders,drivers, sense amplifiers, and input/output circuits. This is done forany of a variety of reasons. For example, the signal delays traversingdown word lines and bit lines which arise from the resistance and thecapacitance of such lines (i.e., the RC delays) may be very significantin a large array. These RC delays may be reduced by subdividing a largerarray into a group of smaller sub-arrays so that the length of each wordline and/or each bit line is reduced. As another example, the powerassociated with accessing a group of memory cells may dictate an upperlimit to the number of memory cells which may be accessed simultaneouslyduring a given memory cycle. Consequently, a large memory array isfrequently subdivided into smaller sub-arrays to decrease the number ofmemory cells which are simultaneously accessed. Nonetheless, for ease ofdescription, an array may also be used synonymously with sub-array torefer to a contiguous group of memory cells having contiguous word andbit lines generally unbroken by decoders, drivers, sense amplifiers, andinput/output circuits. An integrated circuit may include one or morethan one memory array.

In one embodiment, all of the components depicted in FIG. 9 are arrangedon a single integrated circuit. For example, system control logic 330,column control circuitry 310, and row control circuitry 320 are formedon the surface of a substrate and memory array 302 is a monolithic threedimensional memory array formed above the substrate (and, therefore,above system control logic 330, column control circuitry 310 and rowcontrol circuitry 320). In some cases, a portion of the controlcircuitry can be formed on the same layers as some of the memory array302. In one embodiment, any combination of one or more of system controllogic 330, column control circuitry 310, column decoder 312, drivercircuitry 314, block select 316, row control circuitry 320, block select326, array drivers 324, row decoder 322 or analogous circuits can bereferred to as one or more managing circuits.

FIG. 10 depicts a circuit that illustrates one embodiment for readingthe state of a memory cell. To determine which state memory cell is in,a voltage may be applied and the resulting current is measured. A highermeasured current indicates that the memory cell is in thelow-resistivity state. A lower measured current indicates that thememory cell is in the high-resistivity state. FIG. 10 shows a portion ofa memory array including memory cells 450, 452, 454 and 456, which maybe based on the embodiment of FIG. 1. In this example, the steeringelement 4 is a diode. Two of the many bit lines and two of the many wordlines are depicted. A read circuit for one of the bit lines is depictedto be connected to the bit line via transistor 458, which is controlledby a gate voltage supplied by column decoder 312 in order to select orunselect the corresponding bit line. Transistor 458 connects the bitline to a Data bus. Write circuit 460 (which is part of system controllogic 330) is connected to the Data bus. Transistor 462 connects to theData bus and operates as a clamp device that is controlled by clampcontrol circuit 464 (which is part of system control logic 330).Transistor 462 is also connected to comparator 466 and reference currentsupply I_(REF). The output of comparator 466 is connected to a data outterminal (to system control logic 330, a controller and/or a host) andto data latch 468. Write circuit 460 is also connected to data latch468.

When attempting to read the state of the selected memory cell, all wordlines are first biased at Vread (e.g., approximately 2 volts) and allbit lines are at ground. The selected word line is then pulled toground. For example purposes, this discussion will assume that memorycell 450 is selected for reading. One or more selected bit lines arepulled to Vread through the data bus (by turning on transistor 458) andthe clamp device (transistor 462, which receives ˜2 volts+Vt). The clampdevice's gate is above Vread but controlled to keep the bit line nearVread. Current is pulled by the selected memory cell through transistor462 from the V_(SENSE) node. The V_(SENSE) node also receives areference current I_(REF) that is between a high-resistance statecurrent and a low-resistance state current. The V_(SENSE) node movescorresponding to the current difference between the cell current and thereference current I_(REF). Comparator 466 generates a data out signal bycomparing the V_(SENSE) voltage to a Vref-read voltage. If the memorycell current is larger than I_(REF), the memory cell is in thelow-resistance state and the voltage at V_(SENSE) will be lower thanV_(REF). If the memory cell current is smaller than I_(REF), the memorycell is in the high-resistance state and the voltage at V_(SENSE) willbe higher than V_(REF). The data out signal from comparator 466 islatched in data latch 468. In some embodiments, the reference current isbased on the address of the memory cell.

FIG. 3B-FIG. 5 describe technology for programming one or more memorycells by applying one or more pairs of opposite polarity voltageconditions to the memory cell. FIGS. 11 and 12 describe one embodimentfor implementing that programming process in the memory system of FIGS.6-10. FIG. 11 is a flow chart describing the method of programming andFIG. 12 graphically shows the appropriate signals being applied to aportion of the memory array. FIG. 12 shows four memory cells 520, 522,524 and 526. Each memory cell includes a steering element, such as diode524 d, and a reversible resistance-switching material, such as device524 r. In the example of FIG. 12, one of the memory cells, memory cell520, is selected for programming. Two bit lines and two word lines arealso depicted. One of the bit lines connecting to memory cell 520 is theselected bit line and the other bit line is an un-selected bit line. Theword line is connected to memory cell 520 is the selected word line.Other words lines are the un-selected word lines. Note that inembodiments where multiple memory cells are programmed at the same time,there will be multiple selected bit lines. Additionally, if not allmemory cells will be programmed at the same time, there will be multipleun-selected bit lines. There is likely to be multiple un-selected wordlines. In one embodiment, the memory cells are directly connected to thebit lines and word lines. However, the memory cells can also beconnected to the bit lines and word lines indirectly, such as throughone or more other components.

In the programming process of FIGS. 11 and 12, the selected memory cellfirst receives a 2.5 volt forward bias as the first voltage condition ofthe pair of opposite polarity voltage conditions. For the second voltagecondition of the pair of opposite polarity voltage conditions, a rapidtransient of the bit line voltage reverse biases memory cell 520 and thestored charge flows through the reversible resistance-switching materialin a reverse direction relative to the normal forward bias currentdirection of the memory cell, and thereby switches the memory cellresistance into the SET state. This will be described in more detailwith respect to the steps of FIG. 11.

The steps of FIG. 11 provide one example implementation of applying apair of opposite polarity voltage conditions to reversiblyresistance-switching memory cell. That is, the process of FIG. 11 can beperformed as part of step 180 of FIG. 5. In step 502, all word lines arebiased to 5 volts. In step 504, all bit lines are biased 2.5 volts.Steps 502 and 504 can be performed to the same time or at differenttimes. Subsequently, in step 506, the selected bit line is raised to 5volts and a selected word line is lowered to 2.5 volts. When theselected bit line is at 5 volts and the selected word line is at 2.5volts, memory cell 520 (and its diode) are forward biased and receivinga positive voltage (e.g., similar to voltage pulse 112). In step 508,the selected bit line is pulled down to ground as fast as possible toreverse bias of the selected memory cell by 2.5 volts (e.g., similar tonegative voltage pulse 114 of FIG. 3B). The reverse bias releases thestored charge to provide a diode reverse recovery current for the memorycell. The un-selected word lines are biased to about 5 volts to allowthe initial bit line voltage of 5 volts. The un-selected bit lines arebiased at 2.5 volts to allow the selected word line voltage of 2.5volts. Memory cell 524 is biased at 2.5 volts, reverse bias relative tothe diode polarity. Memory cell 526 has a 5 volt reverse bias when thebit line is pulled down to ground, but there is no significant storedcharge in the diode of memory cell 526 before the fast bit transient sothe memory cell is not disturbed and avoids programming. In step 510,the selected word line is raised back to 5 volts. In step 512, theselected bit line is raised back to 2.5 volts, and the process iscompleted. In this embodiment, the pair of opposite polarity voltageconditions include the forward bias of step 506 that immediatelytransitions into the negative bias as step 508. Waveform 530 of FIG. 12shows the signal being applied to the selected bit line. Waveform 532shows the signal being applied to selected word line.

FIGS. 13 and 14 describe one embodiment for implementing thatprogramming process in the memory system of FIGS. 6-10. FIG. 13 is aflow chart describing the method of programming and FIG. 14 graphicallyshows the appropriate signals being applied to a portion of the memoryarray. FIG. 14 shows four memory cells 640, 642, 644 and 646. Eachmemory cell includes a steering element and reversibleresistance-switching material. In the example of FIG. 14, one of thememory cells, memory cell 640, is selected for programming. Two bitlines and two word lines are also depicted. One of the bit linesconnecting to memory cell 640 is the selected bit line and the other bitline is an un-selected bit line. The word line is connected to memorycell 640 is the selected word line. Other words lines are theun-selected word lines.

In step 602 of FIG. 13, all word lines are biased at 2.5 volts. In step604, all bit lines are at ground. In step 606, the selected word line ispulled to ground. In step 608, the selected bit line is pulled to 2.5volts, which causes a forward biasing voltage condition to the diode ofthe selected memory cell. It is during this time that the diodes willstore charge (analogous to positive voltage pulse 112 of FIG. 3B). Instep 610, the selected word line is rapidly pulled to 5 volts, drivingthe selected memory cells 640 to a reverse bias of 2.5 volts. Memorycell 642 will also see a negative 5 volt stress. Memory cell 644 is at−2.5 volts. Before the fast transient of the word line in step 610,memory cells 642 and 646 are at 0 bias and have minimum stored charge,which reduces the chance of half selected memory cells experiencingunintended programmed. In step 612 of FIG. 13, selected bit lines arepulled back down to ground. The waveform for the selected bit line isshown in waveform 650. Waveform 652 shows the signal being applied toselected word line.

FIGS. 15 and 16 depict another embodiment for programming memory cellsusing the scheme of FIG. 3B-FIG. 5 with the memory system of FIGS. 6-10.That is, the process of FIG. 15 is one example implementation ofapplying one or more pairs of opposite polarity voltage conditions to areversibly resistance-switching memory cell as part of step 180 of FIG.5. FIG. 16 shows a portion of a memory array that includes memory cell730, 732, 734 and 736. In this example, memory cell 730 is selected forprogramming. Waveform 750 depicts the signal being provided to theselected bit line. Waveform 752 depicts the signal being applied to theselected word line.

In step 702 of FIG. 15, all of the word lines are biased at 2.5 volts.In step 704, the bit lines are biased to ground. In step 706, theselected word line is pulled to ground. In step 708, the selected bitline is pulsed to 2.5 volts. While the selected bit line is at 2.5 voltsand the word line is at ground, selected memory cell 730 is receiving apositive bias of 2.5 volts (analogous to negative voltage pulse 114 ofFIG. 3B). In step 710, the selected bit line is rapidly pulled down toground and the selected word line is synchronously rapidly raised to 2.5volts thereby providing a reverse bias of −2.5 volts to selected memorycell 730 (analogous to negative voltage pulse 114 of FIG. 3B). Sourcebias lines to the bit line and word line drivers are used to providesynchronous fast pulses on the bit line falling edge and word linerising edge. Source line transients are adjusted to anticipate thetravel time down source lines and the array lines. For example, a sourceline transient for a word line selecting a cell near the end of a bitline would be later than the transient for a selected word lineselecting a memory cell at the near end of the bit line. The addressinformation is used to adjust the transient timing for the variouslocations of bits along the array lines.

The above-described embodiment provide examples of changing thereversible resistance-switching memory cell from the first resistancestate to a second resistance state by applying one or more pairs ofopposite polarity voltage conditions to the reversibleresistance-switching memory cell.

One embodiment of a method for programming a storage system compriseschanging a reversible resistance-switching memory cell to a firstresistance state and changing the reversible resistance-switching memorycell from the first resistance state to a second resistance state byapplying one or more pairs of opposite polarity voltage conditions tothe resistance-switching memory cell after changing the reversibleresistance-switching memory cell to the first resistance state.

One embodiment of a storage system comprises a reversibleresistance-switching memory cell and one or more managing circuits incommunication with the reversible resistance-switching memory cell. Theone or more managing circuits change the reversible resistance-switchingmemory cell to a first resistance state. The one or more managingcircuits change the reversible resistance-switching memory cell from thefirst resistance state to a second resistance state by applying multiplepairs of opposite polarity voltage conditions to the reversibleresistance-switching memory cell.

One embodiment of a method for programming a storage system comprises:(a) applying a pair of opposite polarity voltage pulses to a reversibleresistance-switching memory cell in order to change the reversibleresistance-switching memory cell from a first resistance state to asecond resistance state; (b) verifying whether the reversibleresistance-switching memory cell is in the second resistance state; and(c) repeating steps (a) and (b) if the reversible resistance-switchingmemory cell is not in the second resistance state.

One embodiment of a storage system comprises a reversibleresistance-switching memory cell; and one or more managing circuits incommunication with the reversible resistance-switching memory cell. Theone or more managing circuits perform a forming operation on thereversible resistance-switching memory cell. The one or more managingcircuits change the reversible resistance-switching memory cell to afirst resistance state after performing the forming operation. The oneor more managing circuits change the reversible resistance-switchingmemory cell to a second resistance state after performing the formingoperation by applying one or more pairs of opposite polarity voltagepulses to the reversible resistance-switching memory cell.

For a storage system that includes a set of a first type of controllines, a set of a second type of control lines and a set of non-volatilestorage elements each of which is connected to one of the first type ofcontrol lines and one of the second type of control lines, oneembodiment of a method for programming comprises biasing the set of thefirst type of control lines to a first voltage; biasing the set of thesecond type of control lines to a second voltage that is lower than thefirst voltage; raising a selected control line of the second type ofcontrol lines to a third voltage that is higher the second voltage;lowering a selected control line of the first type of control lines to afourth voltage that is lower than the first voltage; and quickly pullingdown the selected control line of the second type of control lines to afifth voltage that is lower than the second voltage.

One embodiment of a storage system comprises reversibleresistance-switching memory cells including a selected memory cell, aset of a first type of control lines connected to the reversibleresistance-switching memory cells, a set of a second type of controllines connected to the reversible resistance-switching memory cells, andone or more managing circuits in communication with the set of the firsttype of control lines and the set of the second type of control lines.The selected memory cell is connected to a selected control line of thefirst type of control lines and a selected control line of the secondtype of control lines. The one or more managing circuits bias the set ofthe first type of control lines to a first voltage. The one or moremanaging circuits bias the set of the second type of control lines to asecond voltage that is lower than the first voltage. The one or moremanaging circuits raise the selected control line of the second type ofcontrol lines to a third voltage that is higher the second voltage. Theone or more managing circuits lower the selected control line of thefirst type of control lines to a fourth voltage that is lower than thefirst voltage. The one or more managing circuits quickly pull down theselected control line of the second type of control lines to a fifthvoltage that is lower than the second voltage.

For a storage system that includes a set of a first type of controllines, a set of a second type of control lines and a set of non-volatilestorage elements each of which is connected to one of the first type ofcontrol lines and one of the second type of control lines, oneembodiment of a method for programming comprises biasing the set of thefirst type of control lines to a first voltage; biasing the set of thesecond type of control lines to a second voltage that is lower than thefirst voltage; lowering a selected control line of the first type ofcontrol lines to a third voltage that is lower than the first voltage;raising a selected control line of the second type of control lines to afourth voltage that is higher than the second voltage; and quicklyraising the selected control line of the first type of control lines toa fifth voltage that is higher than the first voltage.

One embodiment of a storage system comprises reversibleresistance-switching memory cells including a selected memory cell, aset of a first type of control lines connected to the reversibleresistance-switching memory cells, a set of a second type of controllines connected to the reversible resistance-switching memory cells, andone or more managing circuits in communication with the set of the firsttype of control lines and the set of the second type of control lines.The selected memory cell is connected to a selected control line of thefirst type of control lines and a selected control line of the secondtype of control lines. The one or more managing circuits bias the set ofthe first type of control lines to a first voltage. The one or moremanaging circuits bias the set of the second type of control lines to asecond voltage that is lower than the first voltage. The one or moremanaging circuits lower a selected control line of the first type ofcontrol lines to a third voltage that is lower than the first voltage.The one or more managing circuits raise a selected control line of thesecond type of control lines to a fourth voltage that is higher than thesecond voltage. The one or more managing circuits quickly raise thecontrol line of the first type of control lines to a fifth voltage thatis higher than the first voltage.

For a storage system that includes a set of a first type of controllines, a set of a second type of control lines and a set of non-volatilestorage elements each of which is connected to one of the first type ofcontrol lines and one of the second type of control lines, oneembodiment of a method for programming comprises biasing the set of thefirst type of control lines to a first voltage; biasing the set of thesecond type of control lines to a second voltage that is lower than thefirst voltage; lowering a selected control line of the first type ofcontrol lines to a third voltage that is lower than the first voltage;raising a selected control line of the second type of control line to afourth voltage that is higher than the second voltage; and quicklylowering the selected control line of the second type of control line toa fifth voltage that is lower than the fourth voltage and synchronouslyquickly raising the selected control line of the first type of controllines to a sixth voltage that is higher than the fifth voltage.

One embodiment of a storage system comprises reversibleresistance-switching memory cells including a selected memory cell, aset of a first type of control lines connected to the reversibleresistance-switching memory cells, a set of a second type of controllines connected to the reversible resistance-switching memory cells, andone or more managing circuits in communication with the set of the firsttype of control lines and the set of the second type of control lines.The selected memory cell is connected to a selected control line of thefirst type of control lines and a selected control line of the secondtype of control lines. The one or more managing circuits bias the set ofthe first type of lines to a first voltage. The one or more managingcircuits bias the set of the second type of lines to a second voltagethat is lower than the first voltage. The one or more managing circuitslower the selected control line of the first type of control lines to athird voltage that is lower than the first voltage. The one or moremanaging circuits raise the selected control line of the second type ofcontrol line to a fourth voltage that is higher than the second voltage.The one or more managing circuits quickly lower the selected controlline of the second type of control line to a fifth voltage that is lowerthan the fourth voltage and synchronously quickly raise the selectedcontrol line of the first type of control lines to a sixth voltage thatis higher than the fifth voltage.

The foregoing detailed description has been presented for purposes ofillustration and description. It is not intended to be exhaustive or belimited to the precise form disclosed. Many modifications and variationsare possible in light of the above teaching. The described embodimentswere chosen in order to best explain the principles of the technologyand its practical application, to thereby enable others skilled in theart to best utilize the technology in various embodiments and withvarious modifications as are suited to the particular use contemplated.It is intended that the scope of the technology be defined by the claimsappended hereto.

1. A method for programming a storage system, the storage systemincludes a set of a first type of control lines and a set of a secondtype of control lines, the storage system further includes a set ofnon-volatile storage elements each of which is connected to one of thefirst type of control lines and one of the second type of control lines,the method comprising: biasing the set of the first type of controllines to a first voltage; biasing the set of the second type of controllines to a second voltage that is lower than the first voltage; raisinga selected control line of the second type of control lines to a thirdvoltage that is higher the second voltage; lowering a selected controlline of the first type of control lines to a fourth voltage that islower than the first voltage; and quickly pulling down the selectedcontrol line of the second type of control lines to a fifth voltage thatis lower than the second voltage.
 2. The method of claim 1, wherein: theset of non-volatile storage elements includes a reversibleresistance-switching memory cell that is connected to the selectedcontrol line of the first type of control lines and the selected controlline of the second type of control lines; and the lowering of theselected control line of the first type of control lines to the fourthvoltage and the quickly pulling down the selected control line of thesecond type of control lines to the fifth voltage causes the reversibleresistance-switching memory cell to change from a first resistance stateto a second resistance state.
 3. The method of claim 1, wherein: the setof non-volatile storage elements includes a reversibleresistance-switching memory cell that is connected to the selectedcontrol line of the first type of control lines and the selected controlline of the second type of control lines; the reversibleresistance-switching memory cell comprises a diode in series withreversible resistance-switching material; the lowering of the selectedcontrol line of the first type of control lines to the fourth voltagecauses the diode to be forward biased; and the quickly pulling down theselected control line of the second type of control lines to the fifthvoltage causes the diode to be reverse biased and theresistance-switching material to change from a first resistance state toa second resistance state.
 4. The method of claim 3, wherein: thequickly pulling down the selected control line of the second type ofcontrol lines to the fifth voltage causes the diode to experiencecurrent flow while being reverse biased without operating in a breakdowncondition.
 5. The method of claim 1, wherein: the set of non-volatilestorage elements includes a reversible resistance-switching memory cellthat is connected to the selected control line of the first type ofcontrol lines and the selected control line of the second type ofcontrol; and the reversible resistance-switching memory cell comprises asteering element and reversible resistance-switching material.
 6. Themethod of claim 1, wherein: the set of the first type of control linesincludes word lines; and the set of the second type of control linesincludes bit lines.
 7. The method of claim 1, wherein: the third voltageis a same voltage as the first voltage; the fourth voltage is a samevoltage as the second voltage; the fifth voltage is ground; and theraising the selected control line of the second type of control lines toa third voltage is performed synchronously with lowering of the selectedcontrol line of the first type of control lines to the fourth voltage.8. A storage system, comprising: reversible resistance-switching memorycells including a selected memory cell; a set of a first type of controllines connected to the reversible resistance-switching memory cells; aset of a second type of control lines connected to the reversibleresistance-switching memory cells, the selected memory cell is connectedto a selected control line of the first type of control lines and aselected control line of the second type of control lines; and one ormore managing circuits in communication with the set of the first typeof control lines and the set of the second type of control lines, theone or more managing circuits bias the set of the first type of controllines to a first voltage, the one or more managing circuits bias the setof the second type of control lines to a second voltage that is lowerthan the first voltage, the one or more managing circuits raise theselected control line of the second type of control lines to a thirdvoltage that is higher the second voltage, the one or more managingcircuits lower the selected control line of the first type of controllines to a fourth voltage that is lower than the first voltage, the oneor more managing circuits quickly pull down the selected control line ofthe second type of control lines to a fifth voltage that is lower thanthe second voltage.
 9. The storage system of claim 8, wherein: theselected memory cell includes a steering element in series withreversible resistance-switching material; the lowering of the selectedcontrol line of the first type of control lines to the fourth voltagecauses the steering element to be forward biased; and the quicklypulling down the selected control line of the second type of controllines to the fifth voltage causes the steering element to be reversebiased and the reversible resistance-switching material to change from afirst resistance state to a second resistance state.
 10. The storagesystem of claim 9, wherein: the steering element is a diode; and thequickly pulling down the selected control line of the second type ofcontrol lines to the fifth voltage causes the diode to experiencecurrent flow while being reverse biased without operating in a breakdowncondition.
 11. The storage system of claim 8, wherein: the set of thefirst type of control lines includes word lines; and the set of thesecond type of control lines includes bit lines.
 12. The storage systemof claim 8, wherein: the third voltage is a same voltage as the firstvoltage; the fourth voltage is a same voltage as the second voltage; thefifth voltage is ground; and the one or more managing circuits raise theselected control line of the second type of control lines to a thirdvoltage synchronously with lowering of selected control line of thefirst type of control lines to the fourth voltage.
 13. A method forprogramming a storage system, the storage system includes a set of afirst type of control lines and a set of a second type of control lines,the storage system further includes a set of non-volatile storageelements each of which is connected to one of the first type of controllines and one of the second type of control lines, the methodcomprising: biasing the set of the first type of control lines to afirst voltage; biasing the set of the second type of control lines to asecond voltage that is lower than the first voltage; lowering a selectedcontrol line of the first type of control lines to a third voltage thatis lower than the first voltage; raising a selected control line of thesecond type of control lines to a fourth voltage that is higher than thesecond voltage; and quickly raising the selected control line of thefirst type of control lines to a fifth voltage that is higher than thefirst voltage.
 14. The method of claim 13, wherein: the set ofnon-volatile storage elements includes a reversible resistance-switchingmemory cell that is connected to the selected control line of the firsttype of control lines and the selected control line of the second typeof control lines, the reversible resistance-switching memory cellincludes a steering element in series with reversibleresistance-switching material; the raising the selected control line ofthe second type of control line to the fourth voltage causes thesteering element to be forward biased; and the quickly raising thecontrol line of the first type of control lines to the fifth voltagecauses the steering element to be reverse biased without breaking downand causes the reversible resistance-switching material to change from afirst resistance state to a second resistance state.
 15. The method ofclaim 13, wherein: the set of non-volatile storage elements includes areversible resistance-switching memory cell that is connected to theselected control line of the first type of control lines and theselected control line of the second type of control lines; and theraising the selected control line of the second type of control line tothe fourth voltage and the quickly raising the control line of the firsttype of control lines to the fifth voltage causes the reversibleresistance-switching memory cell to change from a first resistance stateto a second resistance state.
 16. The method of claim 13, wherein: theset of the first type of control lines includes word lines; and the setof the second type of control lines includes bit lines.
 17. The methodof claim 13, wherein: the second voltage is a same voltage as the thirdvoltage; the fourth voltage is a same voltage as the first voltage; andthe second voltage is ground.
 18. A storage system, comprising:reversible resistance-switching memory cells including a selected memorycell; a set of a first type of control lines connected to the reversibleresistance-switching memory cells; a set of a second type of controllines connected to the reversible resistance-switching memory cells, theselected memory cell is connected to a selected control line of thefirst type of control lines and a selected control line of the secondtype of control lines; and one or more managing circuits incommunication with the set of the first type of control lines and theset of the second type of control lines, the one or more managingcircuits bias the set of the first type of control lines to a firstvoltage, the one or more managing circuits bias the set of the secondtype of control lines to a second voltage that is lower than the firstvoltage, the one or more managing circuits lower a selected control lineof the first type of control lines to a third voltage that is lower thanthe first voltage, the one or more managing circuits raise a selectedcontrol line of the second type of control lines to a fourth voltagethat is higher than the second voltage, the one or more managingcircuits quickly raise the control line of the first type of controllines to a fifth voltage that is higher than the first voltage.
 19. Thestorage system of claim 18, wherein: the selected memory cell includes asteering element in series with reversible resistance-switchingmaterial; the raising the selected control line of the second type ofcontrol line to the fourth voltage causes the steering element to beforward biased; and the quickly raising the control line of the firsttype of control lines to the fifth voltage causes the steering elementto be reverse biased without breaking down and causes the reversibleresistance-switching material to change from a first resistance state toa second resistance state.
 20. The storage system of claim 19, wherein:the set of the first type of control lines includes word lines; and theset of the second type of control lines includes bit lines.
 21. Thestorage system of claim 20, wherein: the steering element is a diode;the second voltage is a same voltage as the third voltage; the fourthvoltage is a same voltage as the first voltage; and the second voltageis ground.
 22. A method for programming a storage system, the storagesystem includes a set of a first type of control lines and a set of asecond type of control lines, the storage system further includes a setof non-volatile storage elements each of which is connected to one ofthe first type of control lines and one of the second type of lines, themethod comprising: biasing the set of the first type of control lines toa first voltage; biasing the set of the second type of control lines toa second voltage that is lower than the first voltage; lowering aselected control line of the first type of control lines to a thirdvoltage that is lower than the first voltage; raising a selected controlline of the second type of control line to a fourth voltage that ishigher than the second voltage; and quickly lowering the selectedcontrol line of the second type of control line to a fifth voltage thatis lower than the fourth voltage and synchronously quickly raising theselected control line of the first type of control lines to a sixthvoltage that is higher than the fifth voltage.
 23. The method of claim22, wherein: the set of non-volatile storage elements includes areversible resistance-switching memory cell that is connected to theselected control line of the first type of control lines and theselected control line of the second type of control lines, thereversible resistance-switching memory cell includes a steering elementin series with reversible resistance-switching material; the quicklylowering the selected control line of the second type of control linesto the fifth voltage causes the steering element to be forward biased;and the quickly lowering the selected control line of the second type ofcontrol lines to a fifth voltage causes the steering element to bereverse biased without breaking down and causes the reversibleresistance-switching material to change from a first resistance state toa second resistance state.
 24. The method of claim 22, wherein: the setof the first type of control lines includes word lines; and the set ofthe second type of control lines includes bit lines.
 25. The method ofclaim 22, wherein: the second voltage is a same voltage as the thirdvoltage and the fifth voltage; the fourth voltage is a same voltage asthe first voltage and the sixth voltage; and the second voltage isground.
 26. A storage system, comprising: reversibleresistance-switching memory cells including a selected memory cell; aset of a first type of control lines connected to the reversibleresistance-switching memory cells; a set of a second type of controllines connected to the reversible resistance-switching memory cells, theselected memory cell is connected to a selected control line of thefirst type of control lines and a selected control line of the secondtype of control lines; and one or more managing circuits incommunication with the set of the first type of control lines and theset of the second type of control lines, the one or more managingcircuits bias the set of the first type of lines to a first voltage, theone or more managing circuits bias the set of the second type of linesto a second voltage that is lower than the first voltage, the one ormore managing circuits lower the selected control line of the first typeof control lines to a third voltage that is lower than the firstvoltage, the one or more managing circuits raise the selected controlline of the second type of control line to a fourth voltage that ishigher than the second voltage, the one or more managing circuitsquickly lower the selected control line of the second type of controlline to a fifth voltage that is lower than the fourth voltage andsynchronously quickly raise the selected control line of the first typeof control lines to a sixth voltage that is higher than the fifthvoltage.
 27. The storage system of claim 26, wherein: the quicklylowering the selected control line of the second type of control linesto the fifth voltage and the quickly raising the selected control lineof the first type of control lines to the sixth voltage causes thereversible resistance-switching memory cell to change from a firstresistance state to a second resistance state.
 28. The storage system ofclaim 26, wherein: the selected memory cell includes a diode in serieswith reversible resistance-switching material; the quickly lowering theselected control line of the second type of control line to the fifthvoltage causes the diode to be forward biased; and the quickly raisingthe selected control line of the first type of control lines to thesixth voltage causes the diode to be reverse biased without breakingdown and causes the reversible resistance-switching material to changefrom a first resistance state to a second resistance state.
 29. Thestorage system of claim 26, wherein: the set of the first type ofcontrol lines includes word lines; the set of the second type of controllines includes bit lines; the second voltage is a same voltage as thethird voltage and the fifth voltage; the fourth voltage is a samevoltage as the first voltage and the sixth voltage; and the secondvoltage is ground.